Injuries to the anterior cruciate ligament (ACL) are occurring at epidermic proportions from athletic endeavors. The most common surgical treatment of an ACL deficiency is ACL reconstruction using autogenous grafts. Although reconstructive techniques are commonly employed, graft implantation variables and post-operative rehabilitation programs remain controversial and unknown. Since the goal of an ACL reconstruction is to restore the normal biomechanics of the knee, it is important to understand the strain behavior of the normal ACL. We propose to continue our in vivo investigation of strain in the normal ACL focusing on commonly prescribed rehabilitation activities (Group #1). In addition, we would like to measure the stress and strain in two commonly used autografts (patellar tendon and hamstrings tendon) during ACL reconstructive surgery (Group #2). The group #1 volunteers are patients who are candidates for arthroscopic meniscectomy. After the routine surgical procedure, a Hall effect strain transducer (HEST) will be implanted into the anteromedial band of the normal ACL. Patients will then perform different rehabilitation activities; isotonic contractions, isometric contractions, squatting, biking, and stair climbing. Strain will be determined as a function of flexion angle and compared with analytical predictions of the shear forces at the knee during these activities. The group #2 patients are volunteers who are candidates for arthroscopic reconstruction of the knee. Graft type will be selected randomly. During graft implantation, a tension measuring device (tensiometer) will be used to document graft placement and set initial graft tension. A modified HEST which incorporates a pressure sensor, calibrated to tensile stress, will be attached to the graft. Stress, strain and laxity measurements will be performed to determine the initial tension required to restore normal knee stability. The graft will then be fixed and the surgery completed. The stress-strain biomechanics will then be measured during anteroposterior shear loading and passive flexion-extension of the knee. Stress-strain comparisons will be made between the two graft types and the normal ACL. This will be the first study to determine if the reconstructed ACL restores normal knee biomechanics. By using these experimental methods, the strain biomechanics of the normal ACL will be characterized, the effects of initial graft tension and graft replacement will be evaluated, and the stress-strain biomechanics of different grafts will be determined. The results of the proposed study will assist the clinician in determining the best surgical treatment and post-operative rehabilitation program for patients undergoing reconstructive surgery. These variables determine the long-term success of the grafts and may reduce cartilage degeneration which has been shown to persists after ACL reconstruction.